1. Field of the Invention
The disclosure relates to the field of the manufacture of tires and more particularly to the field of tires comprising compositions with self-sealing properties used as a puncture-resistant layer, and that are intended to fill in the perforations thus making it possible to avoid the tire going flat when in use.
2. Related Art
These self-sealing compositions are defined by their physicochemical properties and are characterized in that they are capable, under the effect of the internal pressure in the tire, of flowing into the flow paths of the air towards the outside, with a view to sealing them and re-establishing airtightness. They must also be specially adapted to retain a good dimensional stability under the effect of temperature variations, gravity forces or centrifugal forces, so as not to modify the distribution of the masses in the tire.
Many compositions capable of producing these effects are described in the prior art, but have never really been able to be developed in pneumatic tires for vehicles, in particular due to lack of stability over time or of effectiveness under extreme operating temperature conditions, or else due to difficulties in the manufacture and/or use of these self-sealing compositions.
Thus, in order to help to maintain a good effectiveness at high temperature, document U.S. Pat. No. 4,113,799 (or FR-A-2 318 042) proposed, as self-sealing layer, a composition comprising a combination of partially crosslinked high and low molecular weight butyl rubbers, optionally in the presence of a small portion of styrene thermoplastic elastomer.
Document U.S. Pat. No. 4,228,839 proposed, as self-sealing layer for a tire, a rubber blend containing a first polymer material that is degraded by irradiation, such as polyisobutylene, and a second polymer material that is crosslinked by irradiation, preferentially a butyl rubber.
Document U.S. Pat. No. 4,426,468 itself also proposed a self-sealing composition for a tire based on crosslinked butyl rubber having a very high molecular weight.
Butyl rubbers have, despite everything, the drawback of exhibiting high hysteresis losses (i.e. a high tan δ level) over a broad temperature range, which drawback has an impact on the self-sealing compositions themselves with a high increase in the hysteresis, and a significant degradation of the rolling resistance of the pneumatic tires.
Therefore, alternatively, it may prove advantageous to use, for the same type of application, self-sealing compositions based on unsaturated diene elastomer (natural rubber) that are described for example in publications U.S. Pat. No. 4,913,209, U.S. Pat. No. 5,085,942 and U.S. Pat. No. 5,295,525. These compositions are characterized by the combined presence of a high content of hydrocarbon resin as tackifier, always greater than 100 parts by weight per hundred parts. In addition, a large amount of liquid elastomer gives a high fluidity to the composition which is a source of other drawbacks, in particular a risk of the self-sealing composition flowing during use at relatively high temperature (typically above 60° C.) frequently encountered during the use of the tires in certain geographical regions.
More promising, a composition free of butyl rubber and of large amounts of hydrocarbon resin and liquid elastomer is described in publication FR 2 955 587 by the applicant. This composition is based on a blend of a polybutadiene or butadiene copolymer elastomer and a natural rubber or synthetic polyisoprene elastomer (the weight ratio between the two elastomers being within a range of from 10:90 to 90:10); on a hydrocarbon resin (between 30 and 90 phr); and on a filler (0 to less than 30 phr).
These compositions simultaneously contain a network of long elastomeric chains of high molecular weight, and a network of short elastomeric chains of low molecular weight. In the crosslinked state, the long elastomeric chains form a network capable of elastically surrounding the shorter elastomeric chains that are therefore more suitable for retaining their plastic properties.
All these compositions are characterized in that, in the non-crosslinked state, and in the crosslinked state, they have a fluidity and a tack that are very high in comparison with the rubber compositions that are conventionally incorporated into the manufacture of tires. This high level of tack does not make it possible to carry out the curing of these compositions in a mould, since removal from the mould after curing is virtually impossible.
However, the pressure and containment conditions, such as are present during curing in a mould, must be respected during the curing of the elastomer-based compositions in order to obtain the expected properties of the material after crosslinking.
Therefore, the use thereof in standard extrusion and assembly processes is the origin of quite a source of problems.
In particular, during the assembly process, the creation and the application on a receiving surface of a profiled element having calibrated dimensional characteristics prove particularly tricky in that the geometry of the profiled element may be significantly deformed under the effect of the mechanical stresses linked to the handling operations and the transfer of said profiled element, such as the debonding of a support, the tensioning during the application, etc.